I'm running Ubuntu 10.04 and yesterday ran updates which included Linux 2.6.32-39-generic. Following the successful installations of the updates I removed the 3 older kernel versions to reduce the size of the Grub boot page.
I then went to image my HDD using Clonzilla, which has been a routine for some years, but my USB external HDD was not recognised. Many attempts were made but to no avail.
The only USB item I could get my PC to recognise is a 4GB imitation memory stick, but it's recognised as usb0 and with the HDD icon appearing on the screen and in the Places directory. There was also a Rubbish Bin appearing in Places after I'd accessed usb0 - with 2 items which exist on the POCKET memory stick!
When I attempted to unmount usb0, a popup screen asked: Do you want to empty the rubbish bin before you unmount? The response to both Yes & No was: Unable to unmount: /media/usb0 is not in the fstab (and you are not root ).
The usb 4Gb memory stick is known as POCKET and appears in Places as the usual memory stick icon and with POCKET as its name, but is not accessable.

Try reinstalling the previous kernel version -38, and choose that at grub menu, to see if it is a problem related to that -39 version.

I think I have read about other problems on some machines with the 2.6.32-39 version. I use a kernel version 2.6.35-32 from the "proposed" update repos on my 10.04, with no problems and certainly my external USB drive shows and I can backup my system to it. Perhaps worth trying that.